1. Field of the Invention
This invention relates to curable organosiloxane compositions. More particularly, this invention relates to organosiloxane compositions that cure by a hydrosilation reaction and contain a novel combination of two curing catalysts. In addition to accelerating curing of the compositions one of the co-catalysts, an organotitanium compound, improves the resistance of cured articles prepared from the compositions to heat-induced degradation.
2. Background Information
The efficacy of organotitanium compounds as heat stabilizers for organosiloxane compositions is reported in the patent and journal literature. Addition of organotitanates or salts of these compounds to peroxide-curable organosiloxane compositions is reported in U.S. Pat. No. 3,734,882, which issued to T. Talcott on May 22, 1973; in Russian patent No. 1,573,022, which issued on Jun. 23, 1990; in Japanese Laid Open Application 49/33950, which was published on Mar. 28, 1974; and in British patent No. 1,538,432.
U.S. Pat. No. 4,102,852, which issued to Delatorre and Beers on Jul. 25, 1978 teaches adding platinum compounds as flame retarding agents to room temperature vulcanizable organosiloxane compositions comprising a silanol-terminated organopolysiloxane, a curing agent containing silicon-bonded alkoxy groups and a chelated organotitanium compound as the curing catalyst.
The preparation of certain reaction products of titanium- , hafnium- or zirconium compounds and liquid triorganosiloxy-terminated methylpolysiloxane fluids together with the use of these reaction products as heat transfer fluids or heat stabilizers for liquid methylpolysiloxanes is taught in U.S. Pat. No. 4,122,109, which issued to Halm on Oct. 24, 1978.
The use of the titanium-containing reaction products described in the Halm patent as heat stabilizers for a finely divided, particulate form of cured silicone rubber is disclosed in U.S. Pat. No. 4,722,957, which issued to J. Braun and S. Smith on Feb. 2, 1988. The silicone rubber is prepared by a platinum-catalyzed hydrosilation reaction involving a vinyl-substituted organopolysiloxane and an organohydrogenpolysiloxane. Preferred vinyl-substituted organopolysiloxane exhibit a viscosity of less than 5 pascal seconds at 25.degree. C. and contain diorganosiloxane, monoorganosilsesquioxane, and triorganosiloxane units.
A disadvantage of using the heat stabilizers described in the Halm patent in a curable organosiloxane composition of the type described in the Braun and Smith patent is the requirement for pre-reacting an organotitanium compound with a liquid dimethylpolysiloxane and adding the resultant reaction product to the curable organosiloxane composition.